It is highly advantageous to provide tube-type heat exchangers with distributors which insure that the descending liquid will be present in a uniform thin film along the inner wall of a heat exchanger.
Reference may be made to Europatent No. 0 231 213 and my aforementioned copending application for the principles of thin-film heat exchange between a liquid descending in each of a multiplicity of tubes from a liquid space above the upper tube sheet, and a fluid which fills the space below the upper tube sheet and surrounds the tubes.
Such heat exchangers can be used effectively, for example, in heat pumps and can comprise upper and lower tube sheets in the holes of which heat exchanger tubes can be mechanically and sealingly fixed by rolling them into a force-fit or by a welding operation.
Below the upper tube sheet, the tubes can be surrounded by a coolant, more specifically, a refrigerant such as Freon, filling the boiler which is defined between the upper and lower tube sheets.
The region above the upper tube sheet forms a supply vessel for the heat-yielding medium.
In the upper end of each tube, a distributor head can be provided as described in my aforementioned copending application which has a conically downwardly widening apron defining at a sharp lower edge of the distributor head, between the latter and the inner tube wall, an annular gap having a radial gap width of, for example, 0.1 to 1 mm.
The tube and the distributor head cooperate to ensure that the downwardly flowing liquid will form a film along the inner wall of the tube.
In the preferred application of such a heat exchanger, namely as a heat pump allowing utilization of the heat content of a liquid such as surface water, it had been the practice heretofore to have the tubes project into the interior of the supply vessel. This, however, posed problems with freezing, especially when the surface water had a temperature only slightly above the freezing point.
In this case, the cooling operation brings the water closer to the freezing point and the limited ability of the heat exchanger structure to yield on the formation of ice has been a serious drawback. The danger of freezing has been avoided in conventional systems by providing a distributor head above the region in which the tube is surrounded by the refrigerant, well above the upper tube sheet.
This has the significant drawback that solids like sludge particles, sand grains or the like suspended in surface water tend to accumulate in the spaces on the upper tube sheet below the upwardly projecting lips of the tubes, thereby requiring cleaning of the apparatus and interruption of the continuity of operation.
It is also a disadvantage that a projecting tube above the upper tube sheet cannot be fixed in place as readily as a tube substantially flush therewith and cannot be provided in as close proximity to other tubes because of the need for clearance in rolling the tubes in place in the tube sheet. Even welding is inconvenient by having the tubes project above the upper tube sheet.